Process for the recovery of magnesium



Patented May 23, 1939 UNITED STATES PROCESS FOR THE RECOVERY OF MAGNESIUM Philipp Balz, Bitterfeld, Germany, assignor, by mesne assignments, to Magnesium Development Corporation, a corporation. of Delaware No Drawing. Application November 28, 1936, Se-

rial No. 113,253. In

' 4 Claims.

This invention relates to a process for the production of magnesium bythe electrolysis of fused magnesium compounds and to electrolytes for use in connection therewith.

replace the magnesium chloride used as molten electrolyte in the electrolytic production of magnesium by solutions or suspensions of magnesium oxide or magnesium oxychloride in fused fluorides, experience up to the present has shown that chlorides were the only electrolytes which furnished satisfactory results on a commercial scale. Nevertheless a certain measure of success can also be obtained when using as electrolytes melts consisting substantially of magnesium fluoride and barium fluoride containing magnesium oxide in suspension. However, this mixture has a considerably higher melting point than magnesium chloride and thus entails high working temperatures in the region of 9.00 to 1000" C. which, owing to the excessive oxidation ofthe magnesium separating out on the surface of the melt give rise to difliculty in recovering the metal.

In considering the possibility of improving this latter method of operation, it appeared that the plan of lowering the solidification point of a fluoride melt by the addition of a chloride or chlorides, was unsuitable, because experience showed that the intrinsically very-low solubility of magnesium oxide in fluorides becomes practicallynegligible in the presence of magnesium chloride, and because it was always assumed that the potential gradient necessary to decompose magnesium chloride is lower than that required to decompose magnesium fluoride so that a preferential electrolysis of the magnesium chloride would always take place, with the liberation of chlorine, which, in contradistinction to fluorine,

40 would be incapable of reacting with the magnesium oxide present in the melt to form fresh electrolyte.

' The present invention is based on the discovery that, in practice, the foregoing assumption does 5 not hold good and that at the temperatures of about 600 C. or more, at which melts of magnesium-chloride and magnesium fluoride can be formed, the potential gradient necessary to decompose magnesium fluoride is very considerably below that necessary to decompose magnesium chloride.

On the basis of this discovery, the invention contemplates an electrolyte for use in the electrolytic production of magnesium which comi prises a fused mixture of magnesium chloride Whereas various attempts have been made to Germany December 4,

and magnesium fluoride in which magnesium oxide is present.

In the electrolysis of this electrolyte at temperatures between about 600 C. and about 800. 0., use is made of a voltage which is insufficient to decompose the magnesium chloride present, but is suflicient to decompose only the magnesium fluoride. By virtue of this composition of electrolyte, and the foregoing conditions of operation, a selective electrolysis of the magnesium fluoride takes place and the fluorine liberated by the electrolysis combines with the magnesium oxide present to form further quantities of fluoride.

The dissociation voltages of magnesium chloride and magnesium fluoride at temperatures between 600 C. and 800 C. are given in the following table:

Temperature MgF, MgCh In carrying out the process of the present invention, use is preferably made of a fused electrolyte containing the magnesium chloride and the magnesium fluoride in approximately eutectic' proportions corresponding to about '70 parts of magnesium chloride to 30 parts of magnesium fluoride.

The electrolysis is preferably effected in cells with iron cathodes and graphite anodes, in which case the separation of the magnesium takes place on the surface of the bath, in accordance with its lighter specific gravity. The anode is preferably located, in known manner, in the bottom of the cell, in order to prevent its destruction by anhydrous magnesium chloride, but not'below the dissociation voltage of magnesium fluoride at the operating temperature, while supplying magnesium oxide to said melt.

2. A process for the recovery of magnesium which comprises subjecting a fused anhydrous melt substantially consisting oi. magnesium fluoride and magnesium chloride in proportions approximately corresponding to the eutectic at temperatures between about 600 C. and about 800 C. to electrolysis with a voltage below the dissociation voltage of anhydrous magnesium chloride, but not below the dissociation voltage o'f magnesium fluoride at the operating temperature, while supplying magnesium omde to said melt in the vicinity of the anode.

'3. A process for the recovery of magnesium which comprises subjecting a fused anhydrous melt substantially consisting oi. magnesium fluoride and magnesium chloride in proportions approximately corresponding to the eutectic at temperatures between about 600 C. and about 800 C. to electrolysis with a voltage below-the dissociation voltage of anhydrous magnesium chloride, but not below the dissociation voltage of magnesium fluoride at the operating temperature, andmaintaining such proportion by supplying magnesium oxide to said melt in the vicinity oi the anode at a rate substantially commensurate with the decomposition of the magnesium fluoride.

4. A process for the recovery of magnesium 10 which comprises subjecting a fused anhydrous melt substantially consisting of magnesium fluoride and magnesium chloride in proportions approximately corresponding to 70 parts by weight of magnesium chloride to 30 parts by 5 weight of magnesium fluoride at temperatures between about 600 C. and about 800 C. to electrolysis with a voltage below the dissociation voltage of anhydrous magnesium chloride, but

not below the dissociation voltage of magnesium 2o 

